Unlike boot-sector viruses, macro viruses are well-suited to spreading across networks. Any time an infected file is opened from a network file server or e-mailed as an attachment, these viruses can further infect other machines. In fact, macro viruses typically spread much more quickly than boot sector viruses, and most of the top 10 viruses today are macro viruses.

Other Viruses While macro viruses are by far the most common network-spread virus, you should also be wary of two other types of network-spread viruses: file infectors and Active-X viruses. File infectors attach themselves to the end of executable files, often overwriting useful program code. Once an infected executable runs on a computer, the virus infects other executable programs. Similar to macro viruses, file infectors can spread via a network, but since executable files are not often distributed via e-mail, these viruses tend to be less contagious (though more damaging because they often overwrite valuable code).

Active-X viruses are also transmitted via a network, but they exploit Web-browser technologies. Many features of current Web sites rely on small programs called applets to be downloaded with the Web page information and automatically executed by the browser. One type of applet, Microsoft's Active-X Controls, may have complete access to the client machine (depending on the security settings in your browser) and potentially could do anything from sending confidential data back down the wire to reformatting the local hard drive.

The intended damage a virus causes varies from nothing (the majority of viruses) to extreme (destroying all data and format information on your hard drives and perhaps even network drives). However, even viruses that don't attempt carnage can cause your machine to operate erratically. A common indication of an active virus is your machine routinely "freezing" or "blue-screening" for no apparent reason.

Protecting Against Known Viruses Investing in some sound antivirus software is the best way to combat this plague. There are two major approaches to virus protection: direct protection and gateway-based protection. With the direct-protection approach, antivirus software is installed on the machine (client or server) to be protected. Gateway-based protection attempts to intercept network-borne viruses as they enter the network (usually at the Internet connection between the firewall and the workgroup), but they can't help with viruses on floppy diskettes.